The present invention relates to an improved method of driving a cathode ray tube (hereinafter abbreviated to CRT) for color display, and in particular to a drive method for synchronizing timings of a point-sequential RGB color signal, applied as a modulation signal to a modulation electrode of a color CRT, with scanning of the CRT.
The method of the invention is directed in particular to a flat configuration color CRT which has been described in the prior art by the assignee of the present invention, having an array of line cathodes for generating a plurality of electron beams utilized to produce respective portions of each scan line of a display image. FIG. 1 is an oblique view illustrating an arrangement of internal components of an embodiment of such a CRT, in which numeral 101 denotes an array of line cathodes each of which extends vertically, the cathodes being disposed at regular spacings along the horizontal direction. Numeral 102 denotes a set of vertical scanning electrodes which are positioned immediately behind the line cathodes 101, with each vertical scanning electrode extending horizontally and with the vertical scanning electrodes being arrayed along the vertical direction with a fixed pitch. The total number of these vertical scanning electrodes is 1/2 of the total number of scan lines per frame of the displayed picture. The function of the vertical scanning electrodes is to enable or inhibit the emission of electrons from the adjacent portions of the line cathodes 101, by applying suitable selection voltages to these scanning electrodes (i.e. voltages with respect to the line cathodes 101). In operation, one vertical scanning electrode at a time is selected by an emission-enabling potential applied thereto (during a corresponding horizontal scanning interval), and modulated electron beams are generated from the corresponding portions of the line cathodes 101 as described in the following, while an emission-inhibiting potential is applied to each of the other vertical scanning electrodes. Thus during each field, the vertical scanning electrodes are successively selected in this way to implement vertical scanning by the electron beams. Control electrodes which are designated as a G1 electrode 103, a G2 electrode 104, and a G3 electrode 105 are successively disposed at specific spacings on the opposite side of the line cathodes 101 from the vertical scanning electrodes 102, for controlling the electron beams produced from the line cathodes 101 by predetermined potentials applied to these control electrodes. Apertures are formed in these electrodes 103 to 105 of respectively predetermined sizes, at positions respectively corresponding to the vertical scanning electrodes 102, through which the electron beams are passed. Vertical focussing and vertical deflection of the electron beams are then performed by electrodes 106 and 107 and a shield electrode 108. Apertures are of course also formed in these vertical focussing and deflection electrodes 106 and 107 for passing the electron beams, and the central axes of these apertures are vertically displaced relative to the central axes of the apertures formed in the G1 to G3 electrodes 103 to 105. Thus, the electron beams can be deflected vertically by variation of a voltage which is applied between the electrodes 106 and 107. This vertical deflection is used to displace the scan lines of one field of each frame with respect to the other field of the frame, to execute interlace scanning.
Horizontal focussing of the electron beams is performed by first, second and third sets of electrodes 109, 110 and 111, which are successively arrayed following the shield electrode 108 such that each electron beam must pass between an opposing pair of electrodes 109, an opposing pair of electrodes 110, and an opposing pair of electrodes 111. Suitable voltages are applied to these electrodes such that an electrostatic lens is formed between each such opposed pair of electrodes 109 and electrodes 110, and each opposed pair of electrodes 110 and 111, for thereby focussing each electron beam in the horizontal direction. To execute horizontal deflection of the beams, for horizontal scanning, a sawtooth waveform voltage is applied between each of the opposed pairs described above of the electrodes 109, 110 and 111. As a result of this application of a horizontal deflection potential to each electron beam as it passes successively between three pairs of opposed ones of the electrodes 109, 110 and 111, a high degree of deflection sensitivity is attained.
Numeral 112 denotes the anode face of the CRT, having formed thereon vertical stripes of fluorescent material (generally referred to as "phosphor" stripes, and so designated in the following) 113, which are arrayed with constant pitch along the horizontal direction, in a pattern consisting of successive red-emission, green-emission, blue-emission, . . . stripes. These will be referred to in the following simply as R stripes, G stripes, and B stripes. As shown, the stripe pattern consists of repetitions of R, G, B, R, G, B, . . . Such a set of successive R, G and B stripes will be referred to as a color trio. Each of these color stripes is separated from the adjacent color stripes by a black stripe 114. A metal back electrode 115 is formed over these phosphor stripes 113.
The operation of this flat configuration color CRT is as follows. Respective electron beams are derived from electrons emitted by the line cathodes 101, with modulation of each of the electron beams (i.e. by respective modulation voltages applied between each of the line cathodes 101 and the G1 electrode 101) being carried out in a similar manner to a conventional CRT. Vertical scanning is performed by applying an emission-enabling selection voltage to successive ones of the vertical scanning electrodes 102, as described above, during each of successive horizontal scanning intervals (referred to in the following as 1 H intervals). Thus, all of the vertical scanning electrodes 102 are successively selected during one vertical scanning interval (referred to in the following as a 1 V interval). During one 1 V interval, the voltage applied between the electrodes 106 and 107 is held constant. Before the next 1 V interval, this voltage is altered, such as to produce a predetermined amount of vertical deflection of the electron beams, as required for field interlace scanning.
During each 1 H interval, a single horizontal scan is performed by each of the electron beams obtained from the line cathodes 101 respectively, across respective portions of a complete horizontal scanning line. Respective point-sequential video signals (each consisting of a sequence of R, G, B, R, G, B, . . . data values) are applied to the line cathodes 101 for modulating the respective electron beams.
It will be apparent that with such a CRT, the timings at which each electron beam falls upon each of the R, G, B phosphor stripes must precisely correspond to the timings at which corresponding R, G and B video data values are applied to the cathode of that electron beam. Any significant deviations between these timing values will result in incorrect color information being produced on the display, e.g. with R phosphor stripes being erroneously driven by G video data, and so on. In this respect such a flat configuration color CRT basically differs from a conventional color CRT which utilizes a shadow mask that is aligned with the phospor stripe or dot array. It can thus be understood that with such a flat configuration color CRT, it is essential to accurately synchronize the horizontal scanning of the electron beams with the point sequential color video signals which are applied to modulate the electron beams.
A prior art drive system for applying video signal drive to such a flat configuration color CRT will be described referring to the general system diagram of FIG. 2(a), and the corresponding waveform diagram of FIG. 2(b). Here, a flat-configuration color CRT 201 is basically divided into an index signal generating section 203 which is utilized in generating an index signal as described hereinafter, and an image display section 202 which produces a display image. The image display section 202 has the configuration described hereinabove referring to FIG. 1, but in this example has 6 line cathodes for generating respectively electron beams, so that the image display section 202 consists of six blocks (referred to the following as scanning blocks) 202a to 202f, each of which functions to produce a specific part (i.e. 1/6th) of each horizontal scan line of the displayed image, and each of which contains a fixed number of color trios. The rate at which these trios are successively scanned during a horizontal scanning interval will be referred to in the following as the trio frequency, which is identical to the frequency of a B timing signal that is described hereinafter. For simplicity of description, only the part of the drive system relating to scanning block 202b (indicated by cross-hatching in FIG. 2(a)) is shown. The index signal generating section 203 has an electrode structure which is identical to that of one scanning block of the image display section 202, i.e. having a single line cathode and corresponding electrodes. The index signal generating section 203 has an array of vertical phosphor stripes alternating with black stripes, with the pitch of these phosphor stripes being preferably equal to the pitch of the color trios of the image display section 202, although it is possible to make the pitch of the phosphor stripes of the index signal generating section 203 equal to 2/3 that of the color trios. It is possible to use the same material for the phosphor stripes of the image display section 202 as that used for the B phosphor stripes of the image display section. However it is preferable to use a short-wavelength emitting material such as P-47 fluorescent material for the phosphor stripes of the index signal generating section, to attain a high signal/noise ratio of the index signal.
During operation of the CRT, the index signal generating section 203 is scanned by electrode beams produced from the corresponding line cathode, in the same way as each scanning block of the image display section 202, with the start of each horizontal scan of the index signal generating section 203 being synchronized (i.e. by a horizontal sync pulse) with each horizontal scan of the image display section 202. The beam current of the index signal generating section 203 is held fixed, and and the light thus emitted from the phosphor stripes of the index signal generating section 203 is received by a photo-electric conversion element 204 to thereby produce an index signal 220, which is utilized as a timing reference signal as described hereinafter. Since the pitch of the phosphor stripes of section 103 is identical to that of the color trios of the image display section 202, the index signal will consist, during each horizontal scanning interval, of a train of pulses having a frequency which is identical to the trio frequency with which successive color trios are scanned.
With a flat configuration color CRT described hereinabove, horizontal scanning will of course not be precisely linear, so that the timings at which the electron beams of the scanning blocks successively fall upon the phosphor stripes will not be of fixed period, i.e. the trio frequency is not constant. The objective of the prior art drive system of FIG. 2 is (for each of the scanning blocks) to use the index signal generating section 203 to derive timing control signals for respectively gating the R, G and B video signals to the line cathodes of the image display section 202, with these timing control signals being synchronized with points in time at which respective phosphor stripes are actually scanned by the electron beams. This is achieved as follows. Firstly, a write mode of operation is entered, to obtain timing data for each of the scanning blocks, selecting these for data derivation one at a time. Normal horizontal and vertical scanning of the scanning block are performed, but with the electrode beams of the currently selected scanning block having a fixed beam current, and with the beam currents electron beams of the other scanning blocks being set below the "black level" value at which light emission begins. Light thus emitted from the B phosphor stripes of the selected scanning block is transferred through a blue filter 205, to be detected by a photo-electric sensor 211A. In practice, it is then necessary to transfer the output signal from the sensor 211A through a waveform shaping circuit 211B, containing a band pass filter and a limiter, in order to obtain a satisfactory waveform of a scanning timing signal 221 which is thus produced from the waveform shaping circuit 211B, as shown in FIG. 2(b). This scanning timing signal 221 is referred to in the following as a B timing signal, and consists of successive trains of pulses produced during respective horizontal scanning intervals, with the pulse timings during each 1 H interval respectively corresponding to timings at which a specific electrode beam falls upon the B phosphor stripes during that scanning interval. The time intervals t.sub.1, t.sub.2, . . . shown in FIG. 2(b), each extending from a leading edge of a pulse of the index signal 220 until the next leading edge of a pulse of the B timing signal 221, are measured by a measurement circuit 206 and resultant time difference values stored in a memory circuit 207A. In this way, the timings at which the electrode beams fall upon the B phosphor stripes are stored in the memory as respective data values, for each of the electrode beams of one scanning block. The above process is then repeated for each of the other scanning blocks.
Subsequently during normal operation in which an image is to be displayed, based on R, G and B video signals (indicated as E.sub.R, E.sub.G and E.sub.B in FIG. 2(a)) the data values that were stored in the memory 207A are read out in synchronism with successive pulses of the index signal 220. A pulse of a regenerated B timing signal 221 is thereby produced by a pulse generating circuit 207B following each pulse of the index signal 220, i.e. after a delay that is determined by the corresponding data value read out from the memory, so that the pulses of the regenerated B timing signal 221 occur after the time intervals t.sub.1, t.sub.2, t.sub.3, . . . following respective index signal pulses. In this way, the B timing signal 221 is effectively regenerated based on the index signal 220, and hence (ideally) consists of pulses occurring at timings which correspond to respective timings at which the electron beams fall upon the B phosphor stripes of the image display section 202. This regenerated B timing signal is inputted to a 3-phase signal derivation circuit 207C, whereby gate timing signals 222, 223 and 224 are derived with reference to the regenerated B timing signal 221, such that signals 222, 223 and 224 mutually differ in phase by 120.degree. and signal 222 coincides in phase with signal 221. Signals 222, 223 and 224 thus indicate the timings at which the electrode beams fall upon the B, G and R phosphor stripes, i.e. they are used to control the transfer of respective B, G and R modulated video signals through gates 208R, 208G and 208B respectively. Resultant output signals from the gates 208R to 208B are added in an adder 209, to produce a point-sequential color video signal that is amplified in an amplifier 210 and applied to the corresponding line cathode of the CRT. The same operation for producing a point-sequential color video signal is performed in parallel for each of the scanning blocks 202a to 202f, to thereby produce a displayed image.
However in order to accurately reproduce an image by a flat-configuration color CRT in the manner described above, it is necessary that any time difference between the index signal and the B timing signal be within a time interval corresponding to electron beam scanning across the pitch (designated in the following as P) of the R, G, B color stripe trios, i.e. the pitch of the B stripes. For this reason, the problem arises that the accuracy of positioning the horizontal focussing and deflection electrodes during assembling the CRT, and the component dimensional accuracy, must be extremely high.
A further problem which arises with such a prior art drive system is that a band pass filter is used in the process of deriving the B timing signal 221 during the memory write operation described above. Since as stated above there is some degree of horizontal scanning non-linearity, variation of the scanning frequency occurs, and hence variations in the input signal frequency applied to this band pass filter. Thus, phase shifts occur in the output signal from the filter, causing the data values that are stored in the memory to be inaccurate and hence preventing accurate generation of timing signals based upon the stored data values, for controlling transfer of video color signals to modulate the CRT.
Yet another problem which arises with the prior art drive system described above is that, to derive the 3-phase timing signals 222, 223 and 224 from the regenerated B timing signal, a band pass filter is used to derive the third harmonic component of that B timing signal, Since each train of B timing signal pulses of a scanning line is preceded by an interval in which no B timing signal pulses occur but a horizontal sync pulse is produced, the phase shift produced by that band pass filter at the start of each pulse train will deviate from that of the remainder of the pulse train. Thus, this is also a factor which prevents accurate timing signals from being produced for controlling application of the color video signals to modulate the CRT.